In the published German patent application DE 10 2013 114 259 A1, an advantageous variant of a hybrid switching assembly is described, in which the current load of the power semiconductor is minimized over time in order to achieve the highest possible life expectancy as well as the lowest possible dimensioning, which is important especially for power contactors in the current range of several hundred amperes. In order to minimize the time of the load current flowing through the power semiconductor, the commutation time is detected by a current transformer, and the current flow through the semiconductor is maintained only until the opening mechanical switching path has reached a sufficient electrical stability. In this short time, the load current is artificially brought to zero by the semiconductor via its control, so that a safe galvanic isolation is achieved after opening both the extinguishing contact- and the isolating contact assembly connected thereto in series.
When designing a switching instrument, so as to ensure functional safety and to achieve long electrical life, one important task is to prevent heat sealing of the switching contacts. Even when using a suitable contact material, as well as a sufficient contact size, in principle, there is a risk that during the switch-on process, a short contact rebound will occur in the contact, especially for switching instruments for currents in the range of a few hundred amps, for which comparatively high contact pressure forces are required. During such a rebound process brief arcing occurs between the minimally open contacts.
In particular at high arc currents, in the region of the foot-points, local melting of the contact surfaces may occur, which then leads to heat sealing of the two contacts in the renewed, directly following contacting.
The tendency for heat sealing increases here, in particular in contacts whose surfaces already have an altered topography due to numerous switching drives affected by electrical arcing. Here preferably in the range of contact peaks there is point-form heat sealing. If it is not possible to break up such heat sealing during the subsequent switch-off process using switching drives, the switching instrument is no longer functional. Basically, this also applies to hybrid switches. For example, during switch-on of the hybrid switching assembly described in the published German patent application DE 10 2013 114 259 A1, if there is heat sealing of so-called extinguishing contacts, commutation to the power semiconductor no longer occurs during the subsequent switch-off process, which is made possible by the mechanical opening of the extinguishing contacts. In the absence of commutation of the load current to the IGBT, the hybrid switch loses its basic function by briefly passing the load current through the semiconductor, to zeroize the latter. Opening the second mechanical contact assembly, which is responsible for galvanic isolation of the hybrid switch, leads to the formation of long-term electrical arcing and thus to the destruction of the switching instrument.
U.S. Pat. No. 3,639,808 describes a protection circuit for a switching contact of a relay having a triac connected in parallel to a switching contact, which is through-connected prior to the closing and after the opening of the switching contact for a short time interval, to avoid the formation of arcing on the switching contact. In particular, according to U.S. Pat. No. 3,639,808, the triac is through-connected approx. 5-15 milliseconds before closing of the switching contact. However, when the currents to be switched are very large, this can result in considerable loading of the triac and thus to an associated lifespan reduction.